1. Field of the Invention
This invention relates to a temperature sensing apparatus. More particularly, this invention relates to temperature sensing apparatus such as those using diffraction gratings and other wavelength-selective elements.
2. Discussion of Prior Art
Bragg diffraction gratings have been used to measure temperature, pressure and strains, particularly in the oceanographic environment (S. W. James et al., Electronics Letters, 32(12), 1996 and J. Jones, OFS 12, p.36-39, 1997), by their incorporation into apparatus which monitor the change in the diffraction condition as the temperature, pressure and strain change. A typical sensing apparatus comprises a light source, a diffraction grating (which diffracts the incident light in a manner dependent on the pressure, temperature and non-hydrostatic stress) and a detector which monitors the light diffracted bu. the grating.
In such an apparatus, changes in the Bragg diffraction condition occur due to variations in pressure, temperature and non-hydrostatic stress. It is, therefore, difficult to discriminate between changes in diffraction condition caused by changes in only one of these variables. This has been a problem when trying to make temperature sensing apparatus, but has been overcome by the use of two gratings which respond differently to changes in pressure and temperature (S. W. James et al., Electronics Letters, 32(12), 1996). Another solution is to use a suitable detection system (J. Jones, OFS 12, p.36-39, 1997). However, both of these methods have disadvantages in that they are expensive and complex.
In accordance with the present invention a temperature sensing apparatus comprises a source of electromagnetic radiation; a wavelength-selective element which interacts with radiation emitted from the source; and a detection system which monitors radiation from the wavelength-selective element; wherein the wavelength-selective element is fixedly mounted on a substantially rigid substrate.
This provides a cost-effective, versatile and simple temperature sensing apparatus.
In one embodiment, the wavelength-selective element is mounted under strain such that the interaction of the wavelength-selective element and the radiation, and the characteristics of the radiation monitored by the detection system, are substantially independent of pressure.
In an alternative arrangement, the wavelength-selective element is fixedly mounted to the rigid substrate such that the wavelength-selective element is substantially strain-relieved between its mounting points.
This provides a temperature sensing apparatus whose response is substantially independent of stress applied external to the mounting points of the wavelength selective element.
The wavelength-selective element may be located in a recess in the substrate or in a cavity within the substrate. This affords physical protection to the element. The cavity is preferably pressure-sealed and the substrate is substantially incompressible. Thus, a cavity filled with air would be at a relatively constant pressure, independent of the external pressure.
The cavity may be at least partially filled with a fluid with a high thermal conductivity. This improves the speed of response of the temperature sensing apparatus.
In a preferred embodiment, the wavelength-selective element is attached to the substrate by means of mechanical clamps. This system is easy to implement.
Alternatively, the wavelength-selective element is attached to the substrate by means of an adhesive such as solder or an epoxy resin. This gives a permanent strong bond and is easy to implement.
Where the wavelength-selective element is mounted under strain, the wavelength-selective element may be embedded in the substrate. This protects the element from physical and chemical damage.
Alternatively, when the wavelength-selective element is mounted under strain, the wavelength-selective element may be partially embedded in the substrate. This gives support to the element and yields a faster response to changes in temperature than is achieved by wholly embedding the element in the substrate.
Preferably, the wavelength-selective element is covered with at least one additional coating of a material with a low bulk modulus. This further reduces sensitivity to changes in pressure if the fibre is attached under strain to the substrate. The coating supports the wavelength-selective element and protects it from physical damage.
The material with a low bulk modulus is preferably an organic polymer, such as polystyrene. Rexolite, Hytrel 5526 or polyurethane 3130. Mixtures of materials could be used. These materials are cheap and easy to deposit.
In one particular embodiment, two or more wavelength-selective elements are arranged so as to further reduce the pressure dependence.
The wavelength-selective element is preferably an in-fibre grating made substantially of silica with a small dopant content. The dopant is preferably chosen from one of germania, phosphorus and aluminium. The germania-doped fibre is cheap and readily available in the electro-optics industry. The use of silica fibres allows the grating structure to be part of the light transmitting fibre.
Preferably, the substrate comprises a metal. Suitably, the metal is chosen from one of tin, lead and antimony. If the wavelength-selective element is attached under strain to the substrate, then these metals have the right physical properties for the apparatus to have a high sensitivity to changes in temperature and a low sensitivity to changes in pressure.
Preferably, the detection system comprises an optical filter and a photodetector. This provides a cheap and simple detection system.
Alternatively, the detection system comprises an interferometric interrogation system, which provides very high sensitivity and fast response.